1. Field of the Invention
The present invention is in the field of non-dispersive infrared (NDIR) gas analyzers of a type typically used to measure the concentration of an undesirable gas so that an alarm can be given when its concentration approaches a preset level. The present invention achieves this result in a simple and elegant manner without the use of moving parts. Typically, such gas analyzers could be used as the basis for fire detectors and ventilation controllers, as well as detectors of toxic or explosive gases.
2. The Prior Art
The NDIR technique utilizes the characteristic infrared absorption bands of gases for the detection of these gases. The term "non-dispersive" as used herein refers to the apparatus used, typically a narrow-band optical or infrared transmission filter instead of a dispersive element such as a prism for isolating for purposes of measurement the radiation in a particular wavelength band that normally coincides with a strong absorption band in the absorption spectrum of a gas to be measured.
One of the earlier NDIR gas analyzers is described in U.S. Pat. No. 3,793,525 to Burch, et al. In their invention, a beam of infrared energy emanates from an infrared source and passes through a sample chamber containing an unknown gas mixture. Before reaching an infrared detector, the beam is passed through one or more narrow bandpass filters, which may be mounted on a filter wheel. Typically, each filter passes only radiation at the characteristic absorption wavelength of a particular gas of interest. Another filter may also be used as a reference channel at a wavelength that does not overlap the characteristic absorption wavelength of any of the gases present in the sample cell. The use of a rotating filter wheel is typical of the earlier gas analyzers.
In U.S. Pat. No. 3,811,776, Blau, Jr. describes another of the early NDIR gas analyzers. His analyzer exemplifies a class of instruments that use, in addition to the gas sample chamber, a reference sample chamber containing the gas of interest and an identical sample chamber evacuated or filled with a gas that is transparent at the wavelength of the absorption band of the gas to be measured. These additional two chambers are alternately moved into and out of the radiation beam. Since the sample chamber is placed in series with these cells, the alternate introduction of the absorbing and non-absorbing cells into the radiation beam creates, respectively, a reference detector signal and a sample detector signal whose ratio is used to determine the gas concentration in the sample chamber. Unlike the invention of Burch, et al. which utilizes two interposed optical filters to create a sample signal and a reference detector signal, the Blau, Jr. configuration takes advantage of the principle of nonlinear absorption by the gas to be measured (as discussed in U.S. Pat. No. 4,578,762 of Wong), in order to create the reference and sample signals.
In U.S. Pat. No. 4,499,379, Miyatake, et al. and in U.S. Pat. No. 4,501,968, Ebi, et al. show a gas analyzer having a sample cell the interior surface of which is a mirror surface. The gas to be analyzed is heated within the sample cell and the entire volume of gas serves as the source of radiation. Thus, the mirror surface on the inside of the sample cell does not appear to be used in bringing the radiation to bear on the detectors. Two detectors are used, and each has its own filter. The radiation is chopped by means of a rotating mechanical chopper.
These earlier NDIR gas analyzers found acceptance as laboratory instruments in a number of fields, but there remained a need for reducing the size and complexity of the instruments.
The next step in that direction was taken by Wong in U.S. Pat. No. 4,694,173. He proposed NDIR techniques that did not require moving parts, such as mechanical choppers. The goal was to render sensors more rugged and compact for use in a host of new applications.
In U.S. Pat. No. 4,567,366 filed Sep. 26, 1983, Shinohara describes a methane sensor that uses a ratio technique, but the sample chamber is quite different from that of the present invention. The sample chamber is composed of a porous sintered metal or a plastic foam, and therefore is not capable of acquiring a mirror-like finish. In U.S. Pat. No. 4,709,150 filed Mar. 18, 1986, Burough, et al. also show a sample chamber that is composed of a porous plastic or a porous sintered metal.
A major step forward in devising a gas analyzer that is extremely compact and has no moving parts is described in U.S. Pat. No. 5,026,992 to Wong. His approach is to use a black body radiation source whose temperature is alternated between two temperatures such as 523 degree K and 723 degree K, typically, at a frequency on the order of 1 Hz. A single filter having two narrow passbands is used along with a single detector. In contrast, Shinohara uses a source having a constant output spectrum. In Shinohara's instrument, the calculated ratio is the ratio of the detected intensity at the absorption band of the gas to be detected to the intensity of the radiation at a different, reference, wavelength. In contrast, in Wong's technique, the calculated ratio is the ratio of the combined intensity at the absorption band and the reference band when the source is at a first temperature to the combined intensity of the two wavelength bands when the source is at a second temperature. The use of the dual temperature source eliminated the need for moving parts such as a mechanical chopper, thereby eliminating the bulky chopper wheel and its heavy electric motor.
A second major step toward reducing the size and cost of gas analyzers is described in U.S. Pat. No. 5,060,508, also to the present inventor, Wong. Wong discovered that by making the gas sample chamber in the form of a tube having a specularly-reflecting inner surface, the radiation introduced at one end of the tube is conducted with high efficiency to a detector at the other end of the tube and that the actual pathlength travelled by the radiation is substantially greater than the physical length of the tube because of the multiple reflections that occur within the tube. The use of this type of sample chamber considerably reduces the physical size of the gas analyzer.
In the present application, the present inventor describes an NDIR gas analyzer that combines a spectral ratioing technique with a light pipe gas sample chamber to achieve an instrument that is compact, inexpensive, and has no moving parts.